Lecture 4
Chapter 2
Soil Morphology
Morphology refers to form, shape, appearance or
structure. As we study soil morphology we find ourselves at a loss for words--so we make
some words up. This lecture is a parallel study of the form and structure of soils and the
words and symbols used to describe those forms and structures.
- Uppercase letters, O, A, E, B, C, and R are used to
designate master horizons. The symbol "O" represents an organic horizon.
Most soils have no O horizon, they are however, common in forests and swamps and other
places where organic production exceeds decomposition. The symbol "A" means
topsoil. Most soils have an A horizon, but it could have eroded off exposing a subsoil
horizon. Some very young soils have not yet had time to form an A horizon. The symbol
"B" means region of accumulation. Salts and clays accumulate below the A horizon
in a B horizon. Most soils have a B horizon. The symbol "E" means eluvial or
washed out. A small fraction of soils have an E horizon, usually between the A and B
horizons. The symbol "C" means parent material, i.e., unconsolidated mineral
material that has not been affected appreciably by weathering, leaching, etc. The symbol
"R" means bedrock, i.e., mineral material that cannot be considered
unconsolidated. It is too hard for roots to penetrate.
- Various special cases exist. If two or more
subhorizons exist within a master horizon, they can be designated by arabic numerals, such
as B1 and B2, indicating a difference between two horizons within a B horizon. If a
discontinuity occurs so that a master horizon is comprised of two or more distinct parent
materials, this can be designated by preceding letters with Arabic numerals, such as 1B
and 2B. Normally "1" before the first B is understood, and is omitted resulting
a B horizon, followed by a 2B horizon. These special cases are not limited to B horizons.
Transition zones might have properties of two master horizons, and can be called AB, BA,
BC, etc. Boundaries between horizons are often described. Boundaries separating horizons
may be smooth, tongued, wavy, indistinct, or have other properties worthy of note.
- Lower-case letters are used to describe the master
horizons. Think of these descriptions as adjectives. For example, a plowed A horizon
may be designated an "Ap" horizon, with "p" indicating that the soil
has been plowed and is therefore not in a natural condition. As another example, a B
horizon dominated by deposited clay is a "Bt" horizon. The "t" stands
for the German word for clay. Many of these lower-case adjectives are used, a few of which
are explained in the book (see Table 2-2 in the textbook). ***A typical soil in a
semi-arid region may have a vertical sequence of horizons such as: Ap/Bt/Ck. In a cool
forest the sequence may be: Oe/E/Bs/Cg. Many possible combinations of horizon sequences
exist.
- Diagnostic epipedons are surface indicators of soil
properties. Epipedon means topsoil, or surface horizon. Some epipedons are common and
important to soil students. These are: mollic (a thick, dark, fertile topsoil, common in
grasslands), ochric (a thin or light-colored topsoil, common in arid lands), umbric (an
acidic, dark topsoil, common in the subtropics), and histic (an organic topsoil, not
common but important for wetlands and other special cases).
- Some specific kinds of subsoils are common and important.
These are: argillic (a clay accumulation, very common in temperate zones), cambic (a
weakly developed B horizon, often found in young soils), natric (a problematic clay
accumulation with high sodium contents, common in the presence of a high water table),
spodic (an acidic accumulation of humus or oxides, common in cool forests growing in sandy
soil, usually found below an E horizon), calcic (an accumulation of CaCO3,
common in arid & semi-arid lands).
- Soils can be degraded or destroyed. Degradation
occurs through nutrient depletion as we break the nutrient cycle by harvesting crops and
removing them from the field. Erosion of soil can be catastrophic as the topsoil blows or
washes away, leaving only lower-quality B horizon as a plant growth medium. Humus levels
can diminish due to decomposition if the soil is used to produce annual crops and is left
bare much of the year. Many pollutants, including salt, can also degrade or destroy a
soil.
- Soil quality can be assessed. We frequently test soil
to determine its suitability for agricultural, environmental or engineering purposes.
Often these tests are on-going as part of a monitoring program. One can test: physical
properties (texture, infiltration, aeration, temperature) chemical properties (organic
matter, nutrients, pH, salinity, clay type), and biological properties (microbial biomass,
mineralizable substrate).
- Soils are mapped as unique individuals. The smallest
individual that can be considered a unique soil is called a pedon. A pedon must be at
least 1 m2 in area. The USDA uses explicit guidelines to differentiate soil
units. They developed a system of Soil Taxonomy in the early 1970s. This system continues
to evolve. The individual mapping units of soils are called soil series and phases of
series. A series has a name like the "Victoria clay". A phase is a subdivision
of a series, used only where necessary in soils mapping.
Students are encouraged to look up the following vocabulary
words in the textbook glossary and to browse the following web site.
Vocabulary
Web site
The NSCSS Pedology Page has lots of useful information
and links to other sites. URL: www.nscss.org/ped.html
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